DARDANELLE, Ark. — As he announced Tuesday evening that he would seek a seat in the U.S. Senate, Rep. Tom Cotton had reason to be happy.

Cotton, who is in his first term in Congress, is well-positioned to mount a competitive challenge to Sen. Mark Pryor, and he might represent Republicans’ best chance to unseat an incumbent Democratic senator in 2014.

But as he spoke to friends, family and neighbors at the community center in his small hometown of Dardanelle, Ark., Cotton did not smile once.

“Make no mistake: This is not a party, and it is not a celebration,” he sternly told the crowd of roughly 250 people, gathered in a room that smelled of barbecue and was dotted with red, white and blue balloons. “This is a mission briefing.”

His political enemy, Pryor, will be a formidable one: the two-term senator has an impressive family legacy in Arkansas politics that still resonates with older voters, who remember when his father, David Pryor, served as a governor, senator and congressman.

But early polling indicates that Cotton will be competitive and has shown him performing comparably to Pryor among prospective voters. Democrats and Republicans both expect the contest could be the most fiercely competitive in the country, with the best chance of a pickup for the GOP.

In Cotton, Pryor will face the most serious challenger in any Senate campaign he has waged; indeed, in 2008, Pryor did not even have a Republican challenger.

But Pryor’s campaign has expected and prepared for a challenging race. Even before Cotton had made his announcement, Pryor’s campaign rolled out a television ad Tuesday morning criticizing Cotton for his votes to oppose a range of bills, including the farm bill and student aid reform. And it hit on what will likely be a motif during the campaign, emphasizing Cotton’s short time in Congress to portray him as overly ambitious.

“Tom Cotton should be running — not for higher office, but from his own record of hurting the people of Arkansas,” a female narrator says in the ad.

Cotton unveiled his response to the latter critique in his speech, arguing that he has served in Congress “long enough to know that Washington needs to change.”

“I don’t have a lot of seniority,” Cotton said. “I don’t think that’s a bad thing these days. But I have served you in different ways.”

As evidence, he touted heavily his military service in Afghanistan and, to a lesser extent, what he has experienced in Congress during the past seven months.

Cotton’s supporters at the rally enthusiastically accepted that rationale, with cheers during his speech — and, afterward, with musings that he might be better qualified for even higher office.

“When you see what he’s done, he’s had a lot more experience than Barack Obama,” said Valerie Biendra of Fayetteville, Ark. “He’s a lot more qualified to be president than Barack Obama is.”

But as he begins his campaign, Cotton will need to explain some of his more controversial votes to Arkansans, especially his opposition to the farm bill.

“For Arkansas people, it’s kind of a big deal,” said Grant Addison, a sophomore at the University of Arkansas and president of the College Republicans there. “They’ll want a reason why he voted against it.”

In his speech, Cotton hinted that he will explain that vote and others by reprising one of the themes that helped him win support from the Club for Growth and his seat in the House of Representatives: independence from the Republican Party, outside interests and the president.

“Whether I have to stand up to Barack Obama or stand up to the leaders of my own party, which I regularly do, I will stand with you and I will stand with Arkansas,” Cotton said.