Phelan, a Beaumont Republican, had previously insisted he had enough votes to thwart a challenge from the right led by state Rep. David Cook of Mansfield.
The two-term Republican caught many Capitol observers off guard when he emerged as the standard-bearer for a coalition that wants to depose Phelan and reshape how the House operates.
From our premium politics newsletter, The Blast: A new ad from Rep. Caroline Harris Davila raises eyebrows in what increasingly appears to be a competitive race.