BOONE, Iowa — Pete Buttigieg refused to comment on the rise of Bernie Sanders in Iowa amid a flurry of fundraising emails warning his supporters about the prospect of Sanders winning the 2020 Democratic presidential nomination.

"I'm focused on my campaign," Buttigieg told reporters in Boone, Iowa, on Monday. "I think I'm better positioned to beat Donald Trump than him or any of my other competitors."

Among the fundraising emails was one with the subject line, "Trump’s team wants Bernie to be the nominee," sent by deputy campaign manager Hari Sevugan. The former mayor of South Bend, Indiana, 38, said he "generally" signed off on such notes if they were made in his name.

"I stand by any of the communications that have come out from my campaign on this," he added.

Sanders is surging in the first-in-the-nation state a week out from when Democrats will gather to caucus on Feb. 3. The Vermont senator, 78, now leads former Vice President Joe Biden, 77, by an average of 3 percentage points, according to RealClearPolitics data. The boost comes at the expense of Massachusetts Sen. Elizabeth Warren, while Buttigieg and Biden split the center-left primary vote.

Buttigieg on Monday also hit Sanders for saying it is "impossible to predict" the cost of his signature policy, Medicare for All.

"Call me simplistic, but I think, when you put forward a plan, you ought to know how to pay for it," the mayor told reporters.

In an interview with CBS News last week, Sanders said "nobody knows" the exact figure required to roll out his version of a government-funded healthcare system but asserted it would be less expensive than the status quo.

A lack of details regarding their respective "Medicare for all" plans has hurt other 2020 Democratic White House hopefuls, including Warren and California Sen. Kamala Harris. Sanders, however, has not come under the same level of scrutiny over the practicality of the platform.

[Read more: 2020 Democrats tiptoe around fraught history with Bernie Sanders]