Thomas won his maiden Tour last year but is set to be a co-leader at Team Ineos after Egan Bernal won the Tour de Suisse in June. Thomas crashed out of that race and has not reached his top form this year.
With Chris Froome already out of the Tour, this year's race is the most open in several years but Porte – a one-time teammate of Thomas and Froome – believes that the Welshman should be fully backed over the next three weeks.
"I think that Geraint Thomas is the clear favourite," Porte told the press in Belgium on Thursday.
"He won it last year and he's probably in better shape this year, too. It's a shame that Chris Froome is not here but it does change everything. It means Thomas now has to be the out-and-out favourite with Ineos. I'd expect them to back him 100 per cent."
As for Porte, he comes into the Tour de France with far less pressure than in previous years. He has the backing of Trek-Segafredo when it comes to an assault on the yellow jersey but the 34-year-old has struggled with health issues for most of the year. His race programme was subsequently altered several times before he managed fifth at the Tour of California and then 11th at the Critérium du Dauphiné – another race that saw him affected by illness.
Now healthy, he arrives at the Tour with hopes of an overall challenge.
"I must admit that I've not had the season that I wanted to have so far but I go into the Tour with far less pressure than in the last few years. That's no bad thing. The form is pretty good and I just need to stay healthy. I'm happy with the team and we've got a good team here. We've got all the bases covered. Sunday in the team time trial, that's where it's all going to start for us. I feel 100 per cent less pressure. 100 per cent," Porte said.
"It hasn't been a slow start to the season because I wanted it that way. My race programme had to change a few times and I kept getting sick but it's nice to turn up at the Tour not running on fumes and trying to eke out the last bit of form that I've had, which has happened in the last few years. I'm under no pressure from the team, other than to take it one day at a time."
In the last two seasons, Porte has been hit by bad luck, crashing out on stage 9 in both 2017 and 2018. This time around, he is hoping to remain in contention until the final week and, with that in mind, he has already gone to see the final three Alpine stages that will define the overall classification in Paris.
"The last three stages in the Alps are where it's going to be decided anyway, so it might not be a bad thing to be coming here a bit underdone," he said.
"This year I've probably done more altitude than I've ever done in my life. Just before here I was at Isola 2000 and we looked at the Alps stages just after the Dauphiné. It wasn't the easiest recon when you come out tired but those stages are always going to be hard after three weeks of racing. The GC is going to be set by then but you don't want to have a bad day and it's going to be hard for one team to control."