Indianapolis mall operator Simon Property Group has offered to buy Santa Monica’s Macerich Co. for $14.4 billion.

The deal would merge the two rivals, which are respective the first- and third-largest shopping mall owners in the country.

Macerich had previously refused talks regarding a possible deal. But Simon, which last year announced it had built up a 3.6 percent stake in Macerich, put in an offer on Monday worth $22.4 billion: $14.4 billion in cash and stock, plus the assumption of more than $6 billion in Macerich debt.

Macerich has about 53 shopping centers, most of them located in California and Arizona. The deal would help Simon, which has properties nationwide but mostly in the East and Midwest, with its westward expansion.

Macerich shares closed the day at $92.76, up 7 percent, while Simon Property shares were flat, closing the day at $180.44.