WASHINGTON — Republicans control the Senate 51-49.
Democrats would need to flip at least two seats to become the majority, which would give them the ability to block President Donald Trump's appointees to federal judgeships and a stronger hand in legislation. But gaining those two additional seats is a tall order.
The 35 Senate races this year include a disproportionate number of Democratic incumbents trying to hang on in conservative states. With 29 seats to defend, compared with just six for the GOP, Democrats will be hard-pressed to avoid losing ground, much less gain any.
Leaving aside some outside chances (a Republican win in New Jersey, a runoff in Mississippi), the key races break down into four categories:
Two opportunities for Democrats
The Democrats' hopes of a majority depend heavily on winning these two races in the Southwest, where the steady increase in the number of Latino voters has improved Democratic prospects.
• Nevada: Jacky Rosen vs. Sen. Dean Heller
Heller, first elected in 2012 after being appointed to fill a vacancy, is the sole Republican senator running in a state Hillary Clinton carried in 2016. He managed to alienate all sides by publicly flip-flopping on repeal of the Affordable Care Act before finally voting in favor. But his chilly relations with Trump have since thawed.
"I didn't like him, he didn't like me, and as we fought and fought and fought, believe it or not, we started to respect each other," Trump said at a pro-Heller rally in Las Vegas. "And then we love each other."
Rosen, a computer programmer and former head of her synagogue in Henderson, was not Democratic officials' first pick to run. Heller has made an issue of her thin political resume and ambitious bid to advance to the Senate before finishing her lone term as one of Las Vegas' representatives in the House.
Nevada has trended Democratic in presidential races but it's been solidly red in off-year elections, when voting falls off dramatically.
Polls suggest the race is a toss-up.
• Arizona: Rep. Kyrsten Sinema vs. Rep. Martha McSally
Republican Sen. Jeff Flake, an on-again, off-again Trump critic, decided to quit rather than face probable defeat in the GOP primary. The result is a contest between two chameleonlike candidates who've undergone significant political makeovers.
In two terms representing a Tucson district evenly split between Democrats and Republicans, McSally forged a reputation as a relative moderate and made clear that although s Republican, she, too, was no fan of Trump. She even declined to say whether she voted for him in 2016. But to win the nomination and appeal to Republican base voters, McSally became an all-in Trump acolyte; the question is whether she went overboard in a state the president carried with less than 50 percent of the vote.
Sinema shed her past as a Green Party liberal after being elected to represent the Tempe area in 2012 and has positioned herself in the Senate race as a nonpartisan problem-solver willing to buck the leaders of both parties. McSally portrays Sinema as a fraud and opportunist. Sinema speaks of her willingness to "learn and grow."
Polls suggest the race is dead even.
The four most endangered Democrats
Even if Democrats win both Nevada and Arizona, the party could lose ground because Republicans have strong chances of knocking off incumbent Democrats in these conservative states.
• North Dakota: Sen. Heidi Heitkamp vs. Rep. Kevin Cramer
Heitkamp's victory in 2012 was something of a political miracle for the Democrats. She won by fewer than 3,000 votes as Mitt Romney, the Republican presidential candidate, carried her state by 19 percentage points. Since then, the state has grown even more Republican; Donald Trump carried North Dakota by 36 points.
Heitkamp has portrayed herself as an independent voice who will vote with Trump when she agrees with him, but oppose him on issues, such as his trade battle with China, that threaten to harm North Dakota. Cramer, a Republican who represents the entire state as its sole member of the House, has campaigned as a down-the-line supporter of Trump.
In this year in which all races are to a large extent a referendum on the president, Heitkamp is the most endangered Democratic senator. Her vote against Justice Brett Kavanaugh's confirmation to the Supreme Court signaled that she felt she had nothing left to lose.
Polls consistently have shown her trailing.
• Missouri: Sen. Claire McCaskill vs. state Attorney General Josh Hawley
In 2012, McCaskill, a Democrat, helped engineer her re-election by running ads in the Republican primary that were designed to help the candidate she thought she could most easily beat in the general election, Rep. Todd Akin. She didn't know how right she was. A short time after winning the GOP primary, Akin declared that in cases of "legitimate rape," women don't become pregnant because "the female body has ways to try to shut that whole thing down." The race pretty much ended there.
This year, McCaskill faces a more legitimate opponent. Republicans have attacked her as a wealthy politician out of touch with an increasingly conservative state. She's fought back by pointing to Hawley's decision to join other Republican attorneys general and sue to overturn the Affordable Care Act's protections for people with preexisting health conditions.
Polls show the race a toss-up.
• Indiana: Sen. Joe Donnelly vs. Mike Braun
Donnelly won in 2012 in large part because of mistakes made by his Republican opponent. This time, he faces a stronger opponent in Braun, a businessman and former state legislator.
Donnelly's campaign illustrates the difficulty of running as a Democrat in a conservative state: He has touted his record of voting with Trump on several issues, but also his support for the Affordable Care Act. Braun has run as a political outsider and accused Donnelly of lacking convictions. "He blows with the wind," Braun said at a recent debate.
Polls have shown Donnelly with a very slight lead.
• Florida: Sen. Bill Nelson vs. Gov. Rick Scott
Since the presidential election of 2000, Florida has produced some of the nation's closest election finishes. Also since 2000, Republicans have repeatedly targeted Nelson, a bland, low-profile lawmaker who nonetheless has won repeated statewide races. Scott, a wealthy former health care executive, spent more than $100 million in his two successful races for governor, and this race could break spending records.
Nelson could get a boost from his party's candidate for governor, Andrew Gillum, who has the charisma Nelson lacks and could encourage a high turnout of minority voters. Hurricane Michael, which hit Florida's panhandle on Wednesday, could also affect the race, giving Scott a lot of exposure, for good or ill, as he heads the state's disaster response.
Polls showed Scott leading during the summer, but more recently have shown Nelson taking a small lead.
Two Democrats surviving, so far, in conservative states
• Montana: Sen. Jon Tester vs. state Auditor Matt Rosendale
Tester has never had an easy race in libertarian-leaning Montana. But he's won election to the Senate twice because he's unadorned and accessible, which matters in a state where voters aren't big on pretenses and know their politicians on a first-name basis.
Trump won Montana in a landslide, and Tester has emphasized his willingness to work across party lines. But his votes against Supreme Court nominees Neil Gorsuch and Brett Kavanaugh have underscored the partisan stakes in the election and rallied Republicans behind Rosendale.
Rosendale fought his way through a tough GOP primary. Democrats dubbed him "Maryland Matt," a reference to his home state, and paint him as a carpetbagging developer and faux Westerner who would rather pave over than protect Montana's public lands. That's a serious charge in a state where hunting and fishing are sacred rites and rights.
Polls suggest a tight race, with a slight edge to Tester.
• West Virginia: Sen. Joe Manchin vs. state Attorney General Patrick Morrisey
Manchin, the most conservative Democrat in the Senate, has repeatedly shown an ability to buck national political trends. He won two races for governor, in 2004 and 2008, in elections where Republican presidential candidates carried the state. In 2012, when Romney captured 62 percent of West Virginia's vote for president, Manchin won re-election to the Senate with 61 percent.
Trump won West Virginia by an even bigger margin than Romney, and Republicans thought Morrisey might end Manchin's streak. But Manchin has hammered the Republican on the same issue that McCaskill has used in her race _ the lawsuit by Republican attorneys general that would end health insurance protections for people with pre-existing conditions. That, and his deep popularity with West Virginia voters, may be enough.
Polls show Manchin with a steady lead.
Two Republicans who might be in danger but probably aren't
Democrats talk a lot about these two races, but shouldn't bet on them.
• Tennessee: Former Gov. Phil Bredesen vs. Rep. Marsha Blackburn
The decision by Sen. Bob Corker, a strong critic of Trump's, to retire signaled the president's political dominance of the Republican Party. Blackburn, a staunch conservative, has tied herself tightly to the president. Too tightly, Democrats hope. They recruited Bredesen, a popular former two-term governor, to run.
Bredesen, a centrist like Corker, has emphasized his willingness to work with Trump. He said, for example, that he would have voted for Kavanaugh. But Republicans have poured money into the state, and its natural political leanings seem to be reasserting themselves.
Polls in the summer showed Bredesen ahead; more recently they show Blackburn leading.
• Texas: Rep. Beto O'Rourke v. Sen. Ted Cruz
In a private meeting for big Republican donors in September, a senior Trump administration official warned that Cruz could be in trouble because voters don't find him "likable." Many Republican senators don't, either. O'Rourke, by contrast, has become a Democratic heartthrob this year, with adoring headlines in national publications, huge crowds at rallies and comparisons to the Kennedys.
Likable or not, however, this is Texas, and Cruz is a Republican, and his party has won 170 statewide races since the last time it lost one, in 1994. Unless he can generate a previously unseen turnout by Latinos, O'Rourke won't break that streak.
Polls have shown a close race, but Cruz has held a steady lead.