Senate Republicans are fighting to save their majority, a final election push against the onslaught of challengers in states once off limits to Democrats but now hotbeds of a potential backlash to President Donald Trump and his allies on Capitol Hill.
Fuelling the campaigns are the Trump administration's handling of the COVID-19 crisis, shifting regional demographics and, in some areas, simply the chance to turn the page on the divisive political climate.
Control of the Senate can make or break a presidency. With it, a reelected Trump could confirm his nominees and ensure a backstop against legislation from House Speaker Nancy Pelosi, D-Calif.
Without it, Joe Biden would face a potential wall of opposition to his agenda if the Democratic nominee won the White House.
In North Carolina, for example, the match-up between GOP Sen. Thom Tillis and Democratic challenger Cal Cunningham, among the most expensive in the nation, is close.
At some point, you put it in the hands of voters, said Dallas Woodhouse, a former executive director of the state's Republican Party.
Republican incumbents are straining for survival from New England to the Deep South, in the heartland and the West and even Alaska.
Overpowered in fundraising and stuck in Washington until just last week to confirm Trump's Supreme Court nominee, they are fanning out some alongside Trump for last-ditch, home-state tours to shore up votes.
With the chamber now split, 53-47, three or four seats will determine Senate control, depending on which party wins the White House. The vice president breaks a tie in Senate votes.
What started as a lopsided election cycle with Republicans defending 23 seats, compared with 12 for Democrats, quickly became a more stark referendum on the president as Democrats reached deeper into Trump country and put the GOP on defense.
Suddenly some of the nation's better-known senators Lindsey Graham in South Carolina, Susan Collins in Maine faced strong reelection threats. Only two Democratic seats are being seriously contested, while at least 10 GOP-held seats are at risk.
I don't see how we hold it, said Chip Felkel, a Republican strategist in South Carolina who opposes the president.
Felkel added: You'd be hard pressed to admit we don't have a Trump problem. The political landscape is quickly changing from six years ago when most of these senators last faced voters. It's a reminder of how sharp the national mood has shifted in the Trump era.
Younger voters and more minorities are pushing some states toward Democrats, including in Colorado, where the parties have essentially stopped spending money for or against GOP Sen. Cory Gardner because it seems he is heading toward defeat by Democrat John Hickenlooper, a former governor.
In more Republican-friendly terrain, the GOP senators must balance an appeal to Trump's most ardent supporters with outreach to voters largely in suburbs who are drifting away from the president and his tone.
Tillis is struggling to gain ground in North Carolina, a presidential battleground, even after Cunningham's sex-texting scandal with an aide.
Arizona could see two Democratic senators for the first time since last century if former astronaut Mark Kelly maintains his advantage over GOP Sen Martha McSally for the seat held by the late Republican John McCain.
A vivid dynamic is in Iowa, a state Trump won in 2016 but is now a toss-up as Sen Joni Ernst struggles to fend off Democrat newcomer Theresa Greenfield.
Ernst wowed Republicans with a 2014 debut ad about castrating hogs but she faced criticism after last month's debate when she stumbled over the break-even price for soybeans.
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