Sen. Rand Paul (R-Kentucky) on the cover of Time. TIME The moment one veteran Republican strategist realized Sen. Rand Paul (R-Kentucky) was flailing as a presidential candidate came when he suddenly decided to take on Donald Trump last week at the first presidential debate.

Challenging Trump's refusal to pledge support for the eventual nominee should have been a moment that earned Paul some credibility among Republicans frustrated with Trump's rise.

But it ended up falling flat.

"It just missed the mark," the strategist said. "He didn't give off a good vibe doing it."

Last fall, Time magazine declared Paul the "most interesting man in politics" and stamped him on its cover. Paul launched his campaign earlier this year pledging that he was a "different kind of Republican."

Four months later, though it's still early in a crowded, fluid race, it's clear that many Republicans want different — but not him.

His campaign is struggling to keep up with his rivals in fundraising. Two of his political allies running an outside super PAC supporting his candidacy were recently indicted on campaign-finance fraud charges. Significant plunges in polling are starting to correspond.

And Trump has an aggressive counterpunch. In a raging statement responding to Paul, the real-estate tycoon took him to task for running for reelection to the Senate at the same time he's campaigning for president.

"I feel sorry for the great people of Kentucky who are being used as a back up to Senator Paul’s hopeless attempt to become President of the United States — weak on the military, Israel, the Vets and many other issues. Senator Paul has no chance of wining the nomination and the people of Kentucky should not allow him the privilege of remaining their Senator," Trump said.

Trump further called Paul's operation a "total mess" and said the senator should leave the race.

"Rand should save his lobbyist’s and special interest money and just go quietly home," he declared. "Rand's campaign is a total mess, and as a matter of fact, I didn’t know he had anybody left in his campaign to make commercials who are not currently under indictment!"

Republican presidential candidate Sen. Rand Paul (R-Kentucky) during the first Republican presidential debate on August 6, 2015, in Cleveland. AP

Polls trickling out after the debate have underscored the challenge ahead for Paul and have led to speculation that he could be one of a handful of GOP candidates who drops out before the Iowa caucuses next year.

Paul now sits just ninth in the first-caucus state of Iowa, according to an average of three polls of the state that have been released this week. One of those polls, from Suffolk University, showed his support plunging to just 2% of likely Republican caucus-goers in Iowa. That put him behind such candidates as Ohio Gov. John Kasich (R), who has been diverting most of his early-state resources to New Hampshire.

Iowa will be especially important for Paul, whose father, Ron, scored with its voters during his previous presidential runs. Matt Kibbe, the director of one super PAC supporting Rand Paul, told Politico in July that the group only had paid staff in Iowa because "it matters so much."

But Paul hasn't shown many signs that he will successfully hold onto his father's voters, let alone significantly expand upon that base. And the indictment of the two allies, which stems from their work on Ron Paul's campaign in 2012, revolves around an alleged money-for-endorsement scheme involving former Iowa State Sen. Kent Sorenson (R), who already pleaded guilty to concealing campaign expenditures.

A Public Policy Polling survey released this week showed Rand Paul with the worst net-favorability rating among all GOP candidates. His standing in Iowa has plunged from 10% in April to just 3% now.

"The biggest loser in the poll is Rand Paul," PPP director Tom Jensen wrote. "Paul's been foundering anyway, and his campaign's ties to the Kent Sorenson mess are probably making things particularly bad for him in Iowa."

Republican 2016 presidential candidates at the start of the first official Republican presidential candidates debate of the 2016 U.S. presidential campaign in Cleveland. Thomson Reuters

Things aren't looking much better in New Hampshire, where Paul was thought to potentially capitalize on its independent-leaning primary electorate, and where his father had two strong showings.

According to a Franklin Pierce University/Boston Herald poll released this week, Paul has seen his favorability rating dip from a positive 57-24 margin in March to a negative 44-45 margin in August. His support in the state also plunged from 13% in March to just 6% now. It put him behind better-funded candidates investing significant resources into the Granite State, like Kasich (12%) and former Florida Gov. Jeb Bush (13%).

During the Republican presidential debate last week, Paul used polls as a major selling point for why Republicans should support his candidacy.

"I'm the only one that leads Hillary Clinton in five states that were won by President Obama. I'm a different kind of Republican," he said in his closing statement.

But the polls he's referring to are from March and April. Perhaps in a sign of his diminished standing, the same polling outlet that found him leading Clinton narrowly in those states — Quinnipiac University — instead measured Clinton's strength against Republican candidates Bush, Wisconsin Gov. Scott Walker (R), and Sen. Marco Rubio (R-Florida) in a poll conducted earlier this month.

"His campaign has been in free-fall," said Greg Valliere, the chief political strategist at the Potomac Research Group, of Paul's debate performance.

"Paul didn't help himself much last night."