Spurs manager Jose Mourinho has made it clear to chairman Daniel Levy that he needs attacking reinforcements to ease the goalscoring burden on Harry Kane and efforts to negotiate a deal with Madrid and Bale have taken place, giving the north London club the edge over United, who are focusing on other priorities at this stage.
Bale has been told he can leave Madrid this summer, seven years after moving to the Bernabeu in an £85.3 million transfer from Spurs.
The La Liga champions are prepared to sell the Wales international for £18m but with Madrid needing to make big cuts to their wage bill, a subsidised loan deal aimed at sharing the cost of Bale's £600,000-a-week salary is also on the table for Spurs and United.
But while sources have told ESPN that United's interest in Bale is serious, manager Ole Gunnar Solskjaer only sees the four-time Champions League winner as a back-up option should a move for top target Sancho fail to succeed.
Bale, who rejected United to sign for Madrid in 2013, is aware of United's interest and their desire to prioritise a move for Sancho.
But despite being viewed as a contingency plan by United, Bale is attracted by the prospect of a move to Old Trafford, with sources having told ESPN he accepts his status as a safety net option in the event of Sancho staying at Dortmund.
United's waiting game over Sancho has given Tottenham the advantage, however, with Bale aware that he can seal a return to the club without waiting for another deal to fall through.
Bale's agent Jonathan Barnett confirmed to ESPN's Alex Kirkland that talks with Spurs are underway.
"Gareth still loves Spurs," Barnett also told BBC Sport Wales. "It's where he wants to be."
Bale scored 56 goals in 203 appearances for Spurs between 2007 and 2013 after arriving from Southampton in a deal worth an initial £5m in May 2007.
And with United still waiting on Sancho, Bale is prepared to re-sign for Spurs in order to ensure he has regular first-team football ahead of the delayed Euro 2020 finals with Wales next summer.