Retired U.S. Air Force officer Evan "Ivan" Thomas said this week that he will seek the seat in a district that sprawls across the northern reaches of the county.
Thomas, 51, joins Democrats Maria Gutzeit, 47, a Santa Clarita business owner and elected water board member, and Lou Vince, 42, a Los Angeles police lieutenant and elected member of the Agua Dulce Town Council, in the race to try to unseat Knight, 48.
Thomas, a former test pilot, also ran in the 2014 primary election for the seat, placing fourth among the eight candidates on the ballot.
He said he would "bring the leadership and experience to tackle our national economic and security issues."
The 25th Congressional District's changing demographics -- registration in the former longtime GOP stronghold is now evenly split between Republicans and Democrats -- have led some observers to believe that the first-termer Knight can be defeated.
He also fared poorly in early fundraising and the National Republican Congressional Committee added him to its list of most vulnerable House incumbents.
But Knight said his fundraising has picked up considerably. He also has strong name identification among voters, having followed his father, the late Pete Knight, in the Legislature.
And although Democrats have caught up with Republicans in registration, GOP voters have historically turned out at the polls in stronger numbers.
Last year, no Democrat made it past the state's open, or "top-two," primary, leaving Knight to compete with fellow Republican and former state legislator Tony Strickland for the post, which opened when Rep. Howard P. "Buck" McKeon announced he would not seek another term.
A spokesman for the NRCC, the political arm of House Republicans, had this to say about the growing field of Knight challengers:
"It looks like Democrats are gearing up for a long and brutal primary in the 25th District," spokesman Zach Hunter said, adding, "Whoever ends up being their nominee will be bloodied, broke, and facing an uphill battle in the general election."
Follow @jeanmerl for the latest in Southern California politics news.