DES MOINES, Iowa -- It was never part of Ted Cruz's strategy to engage Donald Trump in a one-on-one fight this early — but his hand was forced, and that move may now be backfiring.
With just days remaining before the Iowa caucuses, Cruz's campaign is facing a potentially consequential shift in momentum — one that could lift Trump to a caucus night victory and give Marco Rubio a significant boost heading into New Hampshire.
An NBC/WSJ/Marist poll released on Thursday showed an 11-point swing in Cruz's support from three weeks ago, moving Donald Trump from a 4-point deficit to a 7-point edge over Cruz. Rubio now snags 18 percent of the likely caucus-goers' support — and Cruz's campaign went up on Iowa airwaves this week with an ad attacking Rubio on immigration.
And in a new Gallup poll, Cruz's favorability rating among Republicans dropped 9 percentage points to 55 percent, and his unfavorable rating rose 7 percentage points.
It's a significant shift in momentum for Cruz from just two weeks ago, one that's reflected in multiple polls, and one that parallels a strategic shift for the candidate toward attacking Trump. As his standing in Iowa slips, the damage done to the candidate's momentum may be compounded by his move in recent weeks to frame the campaign in Iowa as a two-man race between himself and Trump, setting expectations high.
RELATED: Cruz won't back down on carpet bombing ISIS
For months, Cruz avoided a direct confrontation with Trump, refusing to throw "rocks" back at the front-runner.
A top campaign official told NBC News in early January that Cruz would reverse course on that strategy if he were to lose to Trump in Iowa and New Hampshire, refuting suggestions that the candidate should hit Trump despite well-documented evidence the frontrunner formerly held controversial, more liberal positions. The official said the race would get "messy" if Cruz were to lose the first two states.
This strategy seemed to be working. Throughout December, Cruz was steadily rising in Iowa polls — prompting Trump to direct his fire on Cruz and launch an onslaught of incessant attacks on the surging conservative.
"We'll do 90-minutes, Lincoln-Douglas, mano-a-mano, Donald and me," Cruz roared to a crowd in Fairfield, Iowa, on Tuesday night. Trump had announced just an hour earlier that he would not participate in the GOP debate on Thursday night.
And he began to frame the caucuses as a two-person race, solely between himself and Trump, dismissing the rest of the field as inconsequential.
Cruz promoted himself as Trump's sole competitor, telling a crowd in Waterloo this weekend, "more and more observers are saying this race is coming down to a two-man race between me and Donald Trump."
His campaign manager, Jeff Roe, said after Thursday night's GOP debate, "I mean, clearly, it's a two-man race."
Rep. Steve King, Cruz's national co-chair, opened the door a little more, suggesting, "It's a two-man — potential distant third — race now."
Cruz took his analysis of the race a step further last week, pushing the narrative that the "Washington establishment" had abandoned the likes of Marco Rubio and is "rushing over to support Donald Trump," framing Rubio as no longer a player in the race.
Cruz became the focus of an onslaught of negative attacks from a wide array of Republican figures and interest groups in recent weeks.
Iowa Gov. Terry Branstad said last week that Cruz would be "damaging" to the state. Sarah Palin appeared in Ames to endorse Trump over her former Tea Party star Cruz. A pro-Marco Rubio super PAC began running incessant attack ads on Iowa airwaves.
The Trump campaign has released its own set of attack ads hitting Cruz on immigration. And even a pro-Huckabee super PAC hit Iowa television on Wednesday with an ad questioning Cruz's commitment to his faith and Christianity.
But perhaps the greatest bruise landed to Cruz has been a messaging blitz by a pro-ethanol group in Iowa, America's Renewable Future, which has effectively seeped its anti-Cruz advertising out to farmers across the state. Cruz is now confronted at nearly each of his Iowa campaign stops over his position against the Renewable Fuel Standard, a key economic policy in the state.
Taken together, the attacks appear to have damaged both Cruz's support and his popularity in the state. And a number of his opponents stand to benefit.
Other rival campaigns, including Marco Rubio's team, see potential cracks in Cruz's shell, opening an opportunity to climb and gain greater relevance in the Hawkeye State.
Voters consistently expressed interest in Rubio through the summer and fall, but with a late spree by the candidate on the ground this winter, the Florida senator could continue to chip off late support. The candidates could receive potential boosts by Republicans who intended to caucus for Bush, Christie or Kasich but are now looking for a more viable alternative to challenge Cruz and Trump.
The Cruz camp maintains an optimistic tone.
"[The Rubio campaign has] done nothing to show they are organized or raised money to win beyond [the first voting states]," a Cruz aide told NBC News last week. "So they're not going to [win it]."
The campaigns of Mike Huckabee and Rick Santorum also see a window. The pair intend to travel directly to South Carolina after Iowa and spend the extra week making inroads into the evangelical, conservative electorate there if they foresee fragility in Cruz's grasp on that demographic.
This article originally appeared on NBCNews.com.