"They've been struggling. They need to do something to save the marriage," a source told RadarOnline.com.
The couple just listed their Nashville getaway for a cool $3.45 million, and insiders claim they've been scouting homes in Australia so Kidman can be near her ailing mom, Janelle.
Janelle, 77, was hospitalized in late 2016 with cardiac issues. Nicole lost her dad, Antony, to similar heart problems in 2014.
"Nicole's family is everything to her, and she really wants to be close to her mom," said the source.
The A-listers, both 50, have been sparring over everything from Nic's constant cosmetic procedures to Keith's endless touring and previous substance abuse battles.
"Nicole put her foot down and gave Keith an ultimatum," the source said. "She wants him by her side, and she needs to be with her mom."
Insiders revealed that Urban knows Kidman's serious, especially since her hit show Big Little Lies made her hot again in Hollywood.
"She's taking a huge risk walking away right now," said the source. "But she really wants to keep an eye on Keith, and figure out once and for all where they're headed as a couple."
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