Romney, Michelle Obama headed to Iowa
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Former GOP presidential nominee Mitt Romney and first lady Michelle ObamaMichelle LeVaughn Robinson ObamaBudowsky: Bloomberg-Obama or Klobuchar-Kennedy? The Hill's Campaign Report: New challenges for 2020 Dems in Nevada, South Carolina Obamas share messages wishing each other happy Valentine's Day MORE are heading to Iowa to boost their parties' Senate candidates, a sign of how crucial the Hawkeye State has become to Senate control.

Romney will stump with Iowa state Sen. Joni Ernst (R) on Oct. 12-13. Meanwhile, the first lady will rally with Rep. Bruce BraleyBruce Lowell Braley2020 caucuses pose biggest challenge yet for Iowa's top pollster OPINION | Tax reform, not Trump-McConnell feuds, will make 2018 a win for GOP Ten years later, House Dems reunite and look forward MORE (D) at Drake University in Des Moines on Friday.

The 2012 nominee was an early and ardent Ernst supporter, endorsing her before the primary — and before most other national Republicans had gotten behind her.

Ernst and Braley are locked in a tight race. A new NBC News poll out Sunday found Ernst slightly leading Braley, 46 to 44 percent.

Other marquee politicians in both parties have visited or will visit Iowa, a key early-voting presidential primary state that might also hold the key to Senate control this fall. Vice President Biden is also boosting Braley with a Hollywood fundraiser this evening.

President Clinton and former Secretary of State Hillary ClintonHillary Diane Rodham ClintonRussian interference reports rock Capitol Hill Judge dismisses Nunes' lawsuit against Fusion GPS The Hill's Campaign Report: What to watch for in Nevada MORE already visited Iowa for retiring Sen. Tom HarkinThomas (Tom) Richard HarkinTrump's trial a major test for McConnell, Schumer New Hampshire parochialism, not whiteness, bedevils Democrats Democrats must question possible political surveillance MORE's (D) final Steak Fry, a fundraising event for the party. A number of potential 2016 GOP presidential candidates have stumped for Ernst, most recently Sen. Rob PortmanRobert (Rob) Jones PortmanWhere do we go from here? Conservation can show the way GOP senators offering bill to cement business provision in Trump tax law Mnuchin defends Treasury regulations on GOP tax law MORE (R-Ohio).

Romney has also been mentioned as a potential candidate, though most believe that's unlikely.