Former Colorado House Speaker Andrew Romanoff, a Denver Democrat, on Thursday filed paperwork with the Federal Election Commission concerning a campaign for U.S. Senate.

The filing comes as speculation has ramped up in recent weeks that Romanoff will mount a challenge to Republican U.S. Sen. Cory Gardner, considered by some to be the most vulnerable GOP incumbent senator up for election in 2020.

But a Romanoff representative said Thursday’s filing is an update of previous information concerning a campaign committee dating from Romanoff’s unsuccessful 2010 Senate bid and not specifically related to a 2020 challenge to Gardner.

Democratic operatives tell Colorado Politics that Romanoff and his representatives have been contacting potential supporters and operatives ahead of an anticipated announcement of a campaign.

Colorado Politics has reached out to Romanoff for comment and has not yet heard back.

Romanoff is the president and CEO of Mental Health Colorado, a nonprofit advocacy organization.

The 52-year-old former lawmaker lost a primary against U.S. Sen. Michael Bennet in 2010 and fell short in a bid to unseat U.S. Rep. Mike Coffman, an Aurora Republican, in 2014.

Romanoff raised more than $5 million for his congressional campaign, out-raising Coffman in what was one of the most hotly contested and expensive races in the country that year.

According to the most recent filing with the Federal Election Commission by Romanoff’s still-active campaign committee, he has a negative balance of $66.70 after zeroing out the account earlier this year and then paying some bank charges this summer.

Gardner, who defeated Democratic incumbent Mark Udall by 2 percentage points in 2014, is the only Republican incumbent whose Senate seat is labeled a toss-up ahead of 2020 by election forecasting site Sabato’s Crystal Ball.

A poll released this week by ProgressNow Colorado showed Gardner trailing an unnamed Democratic challenger by 6 percentage points.

Other Democrats have been circling Gardner for what could be Colorado’s marquee race in two years, including U.S. Rep. Ed Perlmutter, former state Sen. Mike Johnston, outgoing House Speaker Crisanta Duran, former U.S. attorney for Colorado John Walsh, former Boulder District Attorney Stan Garnett and outgoing state Rep. Joe Salazar.

Term-limited Gov. John Hickenlooper is also said to be a potential Gardner challenger, though he’s been flirting lately with a run for president and has formed a federal exploratory committee for that race.

According to Gardner’s most recent FEC filing, his campaign has raised $1.8 million through the end of September and had $1 million on hand.

Democrats already running for Gardner’s seat include Lorena Garcia, the executive director of Colorado Statewide Parent Coalition, who announced her bid last month, and activist James Blanton and pharmacist Dustin John Leitzel, both of Denver, who filed statements of candidacy with the Federal Election Commission last year. Scientist Trish Zornio last year formed an exploratory committee to weigh a run.

CLARIFICATION: An earlier version of this story said that Romanoff’s filing Thursday was specifically in relation to a run for U.S. Senate in 2020. But a representative of Romanoff said Thursday’s FEC filing was an update to previously filed paperwork regarding an existing Romanoff Senate campaign committee and was not intended to launch a new campaign for Senate.