A growing number of Democrats are threatening to defy the White House over the national debt, joining Republican calls for deficit cuts as a requirement for consenting to lift the country’s borrowing limit.

The tension is the latest illustration of how the tea-party-infused GOP is driving the debate in Washington over federal spending. And it shows how the debt issue is testing the Obama administration’s clout as Democrats, particularly those from politically competitive states, resist White House arguments against setting conditions on legislation to raise the debt ceiling.

The push-back has come in recent days from Sens. Kent Conrad (D-N.D.), chairman of the Senate Budget Committee, and Joe Manchin (D-W.Va.), a freshman who is running for reelection next year. Sen. Mark Pryor (D-Ark.) told constituents during the Easter recess that he would not vote to lift the debt limit without a “real and meaningful commitment to debt reduction.”

Even Sen. Amy Klobuchar (D-Minn.), generally a stalwart White House ally, is undecided on the issue and is “hopeful” that a debt-ceiling bill can be attached to a measure to cut the federal deficit, said her spokesman, Linden Zakula. Klobuchar is also up for reelection next year.

Months ago it seemed unthinkable that Congress might refuse to raise the borrowing limit. Leaders in both parties agreed that failing to do so would risk a default by the U.S. government, which could send interest rates soaring and cut off Social Security checks, as well as salaries for combat troops.

And although many lawmakers and aides say a bipartisan deal is likely, the insistence on conditions by a small but pivotal group of Democrats suggests that any agreement would almost certainly have to include substantial cuts in the deficit — not just to mollify House Republicans but to satisfy Democrats who could be politically vulnerable on spending issues.

“As catastrophic as it would be to fail to raise our debt ceiling, it’s even more irresponsible to not take this opportunity to own up to our unsustainable spending path,” Sen. Mark Udall (Colo.), another Democrat challenging the White House, said in a statement his office released this week. “If we don’t take action to reduce our deficit spending, Congress will be facing this same debt ceiling vote in the near term – still with no end to our deficits in sight.”

The debate is likely to dominate Capitol Hill as early as next week, when lawmakers return from recess.

The government is expected to reach its $14.3 trillion debt cap in mid-May. Treasury Secretary Timothy F. Geithner has said he can maneuver to avoid a default until early July.

The White House has condemned efforts to attach additional measures to the debt-ceiling issue. Press secretary Jay Carney has called it “a dangerous, risky idea to hold hostage . . . a vote on raising the debt ceiling to any other piece of legislation.”

On Thursday, White House spokeswoman Amy Brundage said legislative leaders in both parties “have been clear that the debt ceiling has to and will be raised to prevent another economic meltdown.” She added that there is also bipartisan agreement about reducing the deficit by trillions of dollars. “If members of Congress act responsibly and try to reach common ground, we can agree to significant deficit reduction without playing reckless politics with our economy,” she said.

Polls show why the debt vote is so difficult for Democrats, who next year are expected to face an uphill battle to retain their narrow Senate majority in an election likely to focus heavily on spending issues.

Just 16 percent of Americans favor lifting the debt ceiling, according to a Wall Street Journal/NBC survey published this month. Nearly six in 10 independents opposed it. Democrats were divided, with nearly half saying they did not know enough to have an opinion.

“If you’re in a red state, if you’re going to be perceived as a moderate, you want to be able to say that you’re for cutting spending,” said Democratic strategist J.B. Poersch, former executive director of the party’s Senate campaign committee.

Senate Majority Leader Harry M. Reid (D-Nev.) signaled this week that he would be willing to negotiate on the terms of the debt-ceiling legislation. He noted his support for a “deficit cap” that would “prove that we’re willing to do something about the debt.”

Often called a “debt fail-safe trigger,” such a cap has been embraced by President Obama as well, to force tax increases and spending cuts if the nation’s debt as a share of the economy has not stabilized by mid-decade. The White House, however, has proposed this measure separate from the debt-ceiling vote. Republicans oppose a deficit cap, arguing that it would mean an automatic tax hike.

Reid, speaking to reporters on a Wednesday conference call, hinted that a deficit cap could be part of a broad debt deal.

“The White House can speak for themselves, but we’re not going to be drawing any lines in the sand,” Reid said, according to a transcript of the call.

Several Democrats, including Conrad, Udall and Klobuchar, have previously resisted raising the debt ceiling without also addressing the deficit. In 2010, as a condition for their support in an earlier vote to lift the ceiling, they insisted on the creation of a bipartisan panel to examine how to cut the deficit.

Conrad said this week that he would again refuse to back any debt-ceiling extension lasting longer than a year without a deficit-reduction plan.

“I’ve voted for short-term extensions, but I won’t vote for a long-term extension,” he told NBC’s “Meet the Press.” “And I won’t do it now unless we have a plan to deal with this debt, because, at the end of the day, this represents a fundamental threat to the economic security of the United States.”

Conrad is a key player in the spending debate, both as the Budget Committee chairman and as one of the members of a bipartisan group called the “Gang of Six” that is hashing out a possible compromise on a 2012 budget plan.

Some lawmakers are pinning their hopes on the Gang of Six negotiations to produce a possible deficit-reduction plan to be paired with the debt-ceiling vote. The group could release its framework as early as next week.

But Conrad’s two Democratic colleagues in the negotiations, Sens. Mark R. Warner (Va.) and Richard J. Durbin(Ill.), said Thursday that their talks should not be tied to the debt limit.

Warner said the debt limit “is not something we should mess with.”

Durbin said any threat not to raise the ceiling was “playing with fire.” He added that he wants to “make certain there is an honest effort to deal with our long-term debt, but linking the two is problematic.”

Manchin kicked off a statewide tour this week by announcing his support for a GOP-backed bill to impose spending caps. The measure is viewed by some lawmakers as a possible point of compromise in the debt-limit debate.

He has been asking audiences for a show of hands to test support for lifting the borrowing cap, with overwhelming numbers expressing opposition.

“You have to pay your debt, but when you find yourself in a hole, you ought to stop digging,” Manchin said Thursday in a statement from his office. He accused the White House and House Republican leaders of “playing partisan games.”

He added, “Only in Washington would people argue that the responsible thing to do is raise the debt ceiling and add trillions of dollars in more debt, without a real and responsible debt fix.”

Staff writers Ben Pershing and Philip Rucker contributed to this report.