WASHINGTON (AP) — Steve Bullock was the lame-duck governor of solidly red Montana, fresh off a failed Democratic presidential bid, when he pivoted and announced he'd challenge Republican Sen. Steve Daines for his seat.

Days after announcing his candidacy last month, the first coronavirus cases appeared in Montana. That shifted the spotlight onto Bullock as he leads the state's pandemic response, leaving Daines in the unusual position of a sitting senator competing for attention.

"I look forward to when I can spend more time thinking about the campaign and doing that work," Bullock said last week. Right now he's making sure "lives are being saved."

The coronavirus, the resulting economic shutdown and President Donald Trump's stumbles addressing the crises have abruptly scrambled this fall's battle for Senate control. Democrats have rising hopes of gaining the minimum three seats they'll need to capture a majority, while Republicans who once banked on a robust economy and improving Trump approval ratings are showing signs of nervousness.

Old GOP assumptions about the political climate "are totally upside down," said long-time GOP pollster Neil Newhouse. "Republicans have to be prepared for an all-out battle, and it's going to be a challenge."

Though a lot can change by November's Election Day, favorable signs for Democrats are evident.

Self-described democratic socialist Sen. Bernie Sanders, I-Vt., ceded the Democratic presidential nomination to former Vice President Joe Biden, a relief to moderate Democratic candidates everywhere.

Democrats out-raised Republicans in nine of 12 higher-profile Senate races in this year's first quarter. Besides Bullock outperforming Daines, Democratic challengers raised more money than GOP Sens. Martha McSally of Arizona, Cory Gardner of Colorado, Susan Collins of Maine and North Carolina's Thom Tillis.

Even Senate Majority Leader Mitch McConnell of Kentucky and South Carolina Sen. Lindsey Graham raised less than long shot Democratic opponents.

Underscoring the direction the political arrow now points, two major GOP committees reserved $100 million for autumn ads in eight states. The spending by the National Republican Senatorial Committee and Senate Leadership Fund is entirely for defending GOP incumbents except for in Michigan, where Democratic Sen. Gary Peters faces well-funded GOP businessman John James.

Polls this month indicate the potential peril for GOP candidates. Fewer than one in four surveyed said they highly trust what Trump says about the outbreak, The Associated Press-NORC Center for Public Affairs Research survey showed. Trump's favorable rating tumbled to 43% in a mid-April Gallup poll, down sharply from 49% two weeks earlier.

A recent Republican National Committee survey of 17 battleground states showed significantly eroded support for Trump since the virus outbreak, a danger sign for GOP Senate candidates. The Republican senatorial committee sent candidates a 57-page memo by O'Donnell & Associates, a strategic communications firm, urging them to blame China for the pandemic and advising, "Don't defend Trump, other than the China Travel Ban — attack China."

"More and more are signs the voters are looking for change, they're looking for greater stability," said J.B. Poersch, who runs the Senate Majority PAC, an outside ally to Democratic leadership.

Illustrating one GOP candidate's approach to Trump, Maine's Collins said it's "not helpful" when he speculates about the virus. Trump mused the next day about injecting disinfectants as a treatment, which doctors warned could kill.

Asked about Trump, Collins noted she didn't back him in 2016, when she's said she wrote in a vote for then-House Speaker Paul Ryan, R-Wis. "I'll work with whomever is elected president," she said during an Instagram live event hosted by Colby College.

Happily for Republicans, their candidates have accumulated more cash than their Democratic challengers in most of the Senate's closest races. With indications the economy's revival may limp slowly, fundraising could be tough all year, helping those already boasting formidable war chests.

Both sides' advisers say stay-at-home orders shackling most Americans' movements generally disadvantage challengers, ominously for Democrats who must oust more incumbents to prevail.

Indefinitely eliminated are attention-grabbing public events and big-dollar fundraisers, forcing a reliance on virtual town halls and money-raising events. Senators usually attract more attention than challengers and have better established fundraising networks.

"All you can do is hold a Zoom meeting your supporters show up at and a few bored reporters," Steven Law, who runs Republicans' Senate Leadership Fund, said of challengers.

Republicans dominate the Senate 53-47 but are defending 23 of the 35 contested seats.

Yet all but two GOP-held seats at stake, Colorado and Maine, are from states Trump won in 2016, mostly easily. Even so, Republican seats in Iowa, Georgia and Kansas are plausible Democratic targets.

Sen. Doug Jones of solidly Republican Alabama is Democrats' most endangered incumbent yet has banked a formidable $15 million. He narrowly won a 2017 special election against Republican Roy Moore, who faced accusations of sexually harassing teenagers decades ago when he was a prosecutor, which he denied.

In Arizona, McSally has blamed China for covering up and not containing the disease. Gun control advocate and former astronaut Mark Kelly, her Democratic challenger, has faulted Republicans for trying to repeal former President Barack Obama's health care law, arguing the virus makes its protections crucial.

In Maine, Collins already faced a difficult reelection following her 2018 support for Brett Kavanaugh, Trump's controversial Supreme Court pick. She's run positive ads thanking local employers like L.L. Bean for producing protective masks, rather than attacking Democratic challenger Sara Gideon, the Maine House speaker.

One spot by the Democratic-allied Majority Forward denounced Collins for saying Trump initially "did a lot that was right." A Collins ad said Gideon allies were "shameless" for politicizing "the worst health and economic crisis in a century."

In Montana, Daines is reaching voters with telephone town halls and Facebook chats. "I'm focused on ensuring the voice of Montana is heard in federal policy," he said.

While Bullock has the spotlight as governor, he's also been pressured by the Trump administration and Daines to ease his statewide stay-at-home order. Bullock let some curbs lapse this week, citing health care and economic realities, not politics.

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Volz reported from Helena, Montana. AP reporters David Sharp in Portland, Maine, Jonathan J. Cooper in Phoenix, Arizona, and Zeke Miller in Washington contributed to this report.