By Lauren Dezenski, CNN

(CNN) -- Iowa's calling!

This Thursday, would-be 2020 presidential candidates are sneaking in a final round of rubbing elbows with Iowa Democrats ahead of the holidays.

On the speaking list at Progress Iowa's sixth annual holiday party: California Rep. Eric Swalwell, businessman Andrew Yang, Oregon Sen. Jeff Merkley and South Bend Mayor Pete Buttigieg.

They're not top-tier 2020 names, but they have certainly been in the pre-campaign churn -- and could use the boost with Iowa Democrats.

Buttigieg set off political insider alarm bells this week when he announced he is not seeking a third term as mayor, potentially opening the door to a 2020 run. Last month when asked if he is running, the 36-year-old replied: "I don't know."

Swalwell was just in New Hampshire on December 15, where he said he is "seriously looking at running for president."

Yang is officially running for president and wants to institute a universal basic income.

Merkley is seeking a change in Oregon state law so he can keep his Senate seat and run for president at the same time and has staffed up in Iowa and New Hampshire, so yeah ... he's essentially running.

Past speakers at this same breakfast have included Vermont Sen. Bernie Sanders, New York City Mayor Bill de Blasio and former Missouri Secretary of State Jason Kander.

Meanwhile, fellow would-be candidates are hanging their stockings while mulling the contours of a run with their families through the coming holiday weekend.

Joe Biden is very much thinking about a presidential run -- and reportedly plans to vet that with family members over the holidays.

Others thinking about it over the holidays: Sen. Cory Booker. The New Jersey senator has said he would "consider running for president" and "take some time during this holiday season to sit with family and close friends and advisers to give it a really good consideration."

In the last week, Julian Castro threw his hat into the ring -- kind of -- by launching a presidential exploratory committee.

The move allows the former HUD Secretary and San Antonio mayor to openly entertain a run for president, but not formally launch his campaign. That announcement -- with whatever the decision may be -- comes January 12. Look for more candidates to launch this kind of in-between exploratory committee to test the presidential waters before a run.

But another Texas Democrat has apparently caught the eye (and imagination) of would-be supporters in Iowa.

Beto O'Rourke seems to be surging in Iowa, according to the new Iowa Poll released by CNN and the Des Moines Register. Castro, meanwhile, clocks in at 1%, alongside fully declared candidate John Delaney.

O'Rourke comes in third at 11% -- pretty impressive considering he's shown the least amount of interest in running compared to the rest of the top-tier contenders.

Coming out on top: Biden at 32%, followed by Sanders at 19%.

And on the Republican side, things may be heating up also. In an interview last week, Trump said he hopes John Kasich and Jeff Flake primary him in 2020. Kasich responded, "careful what you wish for."

A new poll from CNN and SSRS shows while Trump remains the top pick among two-thirds of Iowa Republicans -- nearly the same number (66%) say the state should welcome challengers in the coming Iowa caucuses.