Republicans have known for months that their House majority is in genuine peril. But after another bruising showing in a special election, some in the party are reconsidering the once inconceivable notion of losing the Senate.

It's a sobering possibility, particularly given Republican' confidence not long ago that they probably would increase their Senate edge after the November vote. Far more Democratic senators are facing re-election in states favorable to Republicans than the other way around. That's why the GOP held out hope of expanding its ranks and easing the path for President Donald Trump's agenda.

Yet a Republican congressional victory Tuesday in the Phoenix suburbs has set off new alarm bells.

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Republican Debbie Lesko won the special House election by 6 percentage points, though Mr. Trump captured the district by 21 percentage points in 2016. GOP turnout dropped off, and unlike Republicans' shocking losses in a Pittsburgh-area House race and an Alabama Senate contest, there was no weak GOP nominee to blame in Arizona.

The only explanation was the most worrisome for the GOP: Mr. Trump's presidency is activating Democrats and demoralizing some Republicans and if that trend continues, trouble is ahead.

"The first question is if Democrats can take the undeniably stronger turnout in most of these special elections ... and replicate that in the fall," said Steven Law, a Republican operative running the Senate Leadership Fund, a political action committee at the forefront of Republicans' November strategy. "My guess is they will."

Democrats certainly have a steep climb and must do more than play defense to win the Senate majority. Even if they successfully protect all 26 incumbents — 24 Democrats and two independents who caucus with them — they still would have to pick up two seats. Arizona and Nevada are the most likely.

For every Democratic loss among the 10 incumbents running in states where Mr. Trump won two years ago, Democrats would need to add another Republican pickup. That could leave them dependent on knocking off Republican Ted Cruz in Texas or winning in GOP-dominated Tennessee.

Still, there are signs that seizing the Senate is no longer a pipe dream.

Democratic incumbents are outpacing Republicans in fundraising. Of the 10 Democratic senators running in Trump-won states, nine are among the top 20 campaign fundraisers across all Senate candidates this election cycle. None of their potential Republican opponents has made that cut.

The lone Democratic exclusion, West Virginia's Joe Manchin, ranks 31st, but that still puts him ahead of his potential GOP rivals. In fact, the top Republican Senate fundraisers for the cycle are Roy Moore and Luther Strange, the two Alabama Republicans who vied for the seat now held by Democrat Doug Jones.

Cruz, the Texas senator, tops his Republican colleagues with $9.1 million for his re-election bid. But Democrat Beto O'Rourke, even with his underdog status, has taken in more than $13 million.

In Missouri, where Claire McCaskill has been viewed as among the most vulnerable Democratic senators, the two-term incumbent had more than $11 million in her campaign account this month. That compares with $2 million for the Republican state attorney general, Josh Hawley.

Republicans will have plenty of resources with independent groups and their wealthiest backers paying for advertising and voter outreach. But Democrats' performance among rank-and-file donors is just one more measure of voter enthusiasm.

The Arizona race, in a conservative district northwest of downtown Phoenix, highlighted other Republican concerns.

Republicans tried to turn the tax law into a shield. But the Democratic nominee, Hiral Tipirneni, didn't shy away from hammering Lesko as a lackey for national Republican leaders she said are intent on cutting health care services and Social Security.

The GOP arguments apparently worked well enough for the party to hit its early voting targets in Arizona. But doing that and still winning by only 6 percentage points suggested that the rest of the electorate, including independents, broke solidly for Democrats.

All 10 of the Democratic senators in Trump-carried states who are running for re-election voted against the GOP tax law.

"It wasn't that long ago that Republicans declared that the tax bill was going to solve most or all of their problems," said Democratic pollster Zac McCrary. "Now, even in ruby red Republican area, it's only exacerbated Republican problems, and they're just limping across the finish line in a district like this."

Increasingly bitter Republican primaries also magnify Democrats' early advantages. And beyond being forced to spend precious money now, several Republican primaries have become divisive and could leave the GOP base wounded in November.

In Wisconsin, conservative businessman Kevin Nicholson is accusing his GOP opponent, state Sen. Leah Vukmir, of cuddling up to the party establishment and Gov. Scott Walker. The line of attack may work in a primary, but risks alienating Walker supporters later.

In West Virginia, Republicans are mounting attacks on coal company CEO Don Blankenship, vying to face Manchin, for his role in the deadliest mine disaster in decades.

In Nevada, where Democrat Hillary Clinton won in the 2016 presidential race, Republican Dean Heller has been considered the most vulnerable GOP Senate incumbent. Heller's initial primary challenger, Danny Tarkanian, blasted him for defying Mr. Trump on trying to repeal the 2010 health care law and for resisting admitting he voted for Mr. Trump. Tarkanian has since left the Senate race to run for a House district, but Democrats argue Heller will bear the scars of the attacks as he also worked to mend fences with Mr. Trump, instead of reach out to swing voters.