Reading-born Danyl, who came fourth in the ITV show in 2009, compared the boy band to One Direction, who have gone from strength to strength since finishing third in the seventh series.
Union J have survived two sing-offs, and Danyl said: "No one who has ever come in the bottom two has won the show but Union J are getting better and better. One Direction never showed their true potential and now look at them. I think Union J can do the same, and now District 3 have gone, the boy band vote is no longer split."
Danyl wants James Arthur to win but believes it will be Jahmene or Christopher Malone, who he described as "dated". He added: "People said when Ella went, it's a fix, but they should have voted."
Kathryn Brown, mum of Union J's Josh Cuthbert, said the boys felt their time was up last weekend, when one of the favourites, Ella Henderson, was sent home, and added local support is vital to their success.
Kathryn, who works at Reading recruitment agency Delaney Browne, said: "We were sat right in front of Ella's parents and felt their anguish. Union J and Ella had become great friends - I think she looked upon the boys as her big brothers.
"Josh's sister Victoria was also very upset, because she has got to know Ella over the past few weeks. Ella told her that she would like to adopt her as her sister."
The five acts left in X Factor now sing two songs. See Saturday's show on ITV1 at 8pm to watch them perform and for details on how to vote.