As the first votes in the Republican presidential race approach, Rep. Ron Paul has become a serious force with the potential to upend the nomination fight and remain a factor throughout next year’s general-election campaign.

Although few think the congressman from Texas has a realistic shot at winning the GOP nod, he has built a strong enough base of support that he could be a spoiler — or a kingmaker.

In a muddled field, Paul could win the Iowa caucuses. While other candidates have been hesitant to commit to the state or have had trouble sustaining their initial bursts of support, Paul has been methodically building an organization and a growing corps of followers.

Over the past week, he has spent more than $600,000 on attack ads that are cutting into support for a fellow front-runner, former House speaker Newt Gingrich (Ga.). And Paul has built an organization that will allow him to remain in the race well beyond the early-voting states and amass convention delegates.

Perhaps most fearsome to Republican leaders is Paul’s refusal to rule out a third-party presidential bid that would steal votes from the Republican nominee and make President Obama’s path to reelection considerably easier.

A new Washington Post-ABC News poll, for instance, indicates that Obama and former Massachusetts governor Mitt Romney would be locked in a dead heat in a one-on-one contest. But in a three-way race with Paul, Obama would hold a wide advantage. The survey also suggests that Paul on his own would pose at least as much danger to Obama as Gingrich would.

“The reality is Ron Paul is poised to become a major figure in the Republican Party if his momentum continues and he’s able to win in Iowa,” said GOP strategist Steve Schmidt, who managed the campaign of Sen. John McCain (Ariz.), the party’s 2008 nominee. “The open question is: How much durability does he have over the balance of the race?”

Paul’s campaign did not respond to requests for comment.

The congressman’s libertarian views and longtime opposition to increased federal spending and government interference have inspired a committed following of activist supporters who turn out for rallies and organize online.

Until now, though, rival candidates and Republican leaders have largely ignored Paul, considering him more of a political eccentric than a viable opinion leader, much less a credible presidential candidate.

Many Republicans have long been uneasy with Paul’s views on foreign policy, which fall far outside the GOP mainstream. He opposed the Iraq war, wants to pull the military out of Afghanistan and says conservatives are overstating the nuclear threat from Iran to start a war in the region. He opposes foreign aid, including support for Israel — a point of particular concern within the party — and has said that U.S. actions overseas helped spur the terrorist attacks of Sept. 11, 2001.

After his 2008 presidential bid, Paul wasn’t even granted a spot at the Republican National Convention. He held his own shadow gathering instead.

Now, party officials and other candidates are being forced to reckon with him as a purveyor of policy and, in some respects, the leader of a political movement that the GOP would like to harness.

A major factor is Paul’s relative strength with younger voters, who make up the core of his energetic and growing activist base. In the new Post-ABC survey, Paul is best positioned to cut into Obama’s support among young voters, with 44 percent of registered voters who are younger than 40 backing him in a hypothetical matchup against Obama, while Romney wins 41 percent and Gingrich gets 37 percent.

“Ron Paul is the most consequential guy running for president,” said Grover Norquist, an anti-tax activist and Republican organizer. “All the other guys are basically saying the same things, and one gets to be the nominee. But Ron Paul has changed the nature of the modern Republican Party and brought into it discussions not only of non-interventionist overseas policy but monetary policy.”

Norquist addressed Paul’s alternative 2008 convention and calls it “one of the McCain era’s tactical errors” not to embrace the congressman and his supporter base that year — in effect discounting a potentially energized group of campaign volunteers. He said Paul, unlike his rivals, was drawing new people to the GOP, just as Pat Robertson helped lure millions of evangelical voters into the party with his 1988 presidential bid and the tea party movement attracted more activists in 2010.

“I hope to be there to watch [Paul] speak from the main stage in prime time” at the 2012 GOP convention, Norquist said.

Paul’s organization is expected to be a key advantage in dealing with a new set of Republican Party rules that allow for proportional awarding of convention delegates in many states. GOP strategists said his campaign is studying the rules and mapping a plan to collect hundreds of delegates, even by finishing second or third. That could give Paul a prominent speaking slot, or a central role if there is a brokered convention.

The Paul rise underscores the volatile nature of today’s Republican Party, which through its presidential nominating contest is struggling to balance the ideological views of its newly energized tea-party base with a deep desire to find the best candidate to defeat Obama.

If Romney is the pragmatic choice for voters who are worried about electability, Paul is the pick of a certain breed of ideological purists who have grown skeptical of both parties. And other contenders who might have tapped into that strain — Herman Cain or Michele Bachmann, for instance — have fallen back or stepped aside.

In the short term, Paul’s presence seems most beneficial to Romney.

The congressman’s TV ads in Iowa, combined with an assault on Gingrich from a “super PAC” backing Romney, have eroded Gingrich’s lead in that state. A Gingrich win there would be damaging to Romney, but a Paul victory would help deplete Gingrich’s challenge and help Romney shore up his status as the inevitable nominee.

Part of Paul’s success is that he has largely avoided the kind of scrutiny and campaign attacks that have been heaped upon other top-tier candidates considered more viable White House contenders.

That may change. An extensive Weekly Standard piece published this week details missives published in a subscription newsletter bearing Paul’s name that critics say are racist and anti-Semitic. Paul has said that he did not write the newsletters and that he does not agree with the content.

On Tuesday, Gingrich offered delicate criticism designed to tap into doubts about Paul’s views on foreign policy.

“National security really matters,” Gingrich responded when asked by a reporter about Paul. “Iran really matters. The fact that bad guys attacked the U.S. on 9/11 really matters. People need to take seriously when they go into the caucuses the issue of national security.”

Staff writer Amy Gardner contributed to this report.