Former prime minister Ehud Barak joined those backing Labor MK Colette Avital (Labor) for president on Monday.
A day earlier, MK Ami Ayalon, Barak's archrival for Labor's leadership, announced his support for Shimon Peres, Kadima's candidate.
Barak and Ayalon, as contenders for party leadership, carry weight among their respective supporters within the party.
Peres, apparently, is still supported by many in Labor, the party he deserted more than a year ago for Kadima. This has placed Avital in the uncomfortable position of running against Peres - a rival who is not a Labor member, but whose confidants are working behind the scenes to have the party's central committee nominate him for president.
Labor's constitution enables the nomination of a candidate from outside the party. Although the Labor faction has already approved Avital's candidacy, the central committee must ratify the move. This is being held up in part because party member Michael Bar Zohar has submitted his own candidacy.
Since Bar Zohar is Peres' biographer, his candidacy is seen by Avital as an attempt by Peres' camp to play for time and stall her nomination by the central committee, which convened on Monday.
Recent surveys have convinced Ayalon that Peres still wields considerable clout among party veterans, Arabs and farming communities. Confidants believe that supporting Peres would earn Ayalon points from those sectors in the Labor primaries.
If Avital rescinds her candidacy, Peres' chances against MK Reuven Rivlin (Likud) would increase.
Earlier this week, Avital accused Ayalon of stabbing her in the back by supporting a man who quit Labor, causing it great political harm and taking six Knesset seats in the last elections.