Michael Fallon and Patrick McLoughlin are backing the home secretary.
Over 70 MPs now support her, including cabinet ministers Chris Grayling, Justine Greening and David Mundell.
Meanwhile, fellow contender Michael Gove is to make the first speech of his campaign to become Tory leader, after announcing his candidacy on Thursday.
Rivals Stephen Crabb, Andrea Leadsom and Liam Fox are also in the running to lead the Conservative party and become UK prime minister.
