Top Democrats are gearing up for President Obama's expected executive action on immigration by arguing his Republican predecessors also made policy by decree.

But a review of steps taken by former Presidents Ronald Reagan and George H.W. Bush -- whose executive actions Democrats have cited as precedent -- shows they only used their power to expand laws recently passed by Congress, not to impose new laws.

By contrast, what Obama is expected to do would go far beyond his predecessors' actions. And Obama's expected executive actions would not be rooted in any recently passed law.

The following is a timeline of the actions pursued by Reagan and Bush:

— In 1986, Congress approved the Immigration Reform and Control Act (IRCA). It made illegal immigrants eligible for temporary legal status, if they had been living in the U.S. since 1982, paid a fee and demonstrated “good moral character.” As a result, about 2.7 million immigrants were awarded green cards.

However, the law excluded the spouses and children of those who could benefit under IRCA if they did not meet the law’s requirements.

— In early 1987, Congress introduced legislation to legalize these family members. Their efforts were not successful.

It was after this failed attempt that Reagan granted a deportation reprieve for children under 18 who were living in a two-parent household where both parents got amnesty – or living with a single parent who got amnesty. This, too, was limited, as a child living in a two-parent household where one parent was not eligible also was not eligible. Spouses were not eligible either. Reagan’s executive action expanded the number of eligible people only nominally.

— After Reagan’s term was over, Bush was left with the issue of the remaining family members who were not eligible under IRCA. Though the Senate passed legislation in the summer of 1989, the House didn’t act on the Senate’s bill.

Using the Immigration and Naturalization Service (which was dissolved in 2003 after the creation of the Department of Homeland Security), Bush instituted a "family fairness" policy, which protected all spouses and children of those legalized under IRCA.

Congress soon followed suit, however. By October of 1990, Congress passed the Immigration Act of 1990, including a permanent "Family Unity" provision. The law expanded Bush's administrative fix to include children under 21. In total, roughly 1.5 million individuals related to those already legalized under IRCA benefited from the policy approved by Bush, and later passed by Congress.

— By contrast, Obama is looking at potentially enacting his own policy that gives a reprieve to millions. The president first approved a deportation reprieve in 2012 for some illegal immigrants who came to the U.S. as children. But according to a draft document, he’s considering drastically expanding that program to include even more who came to the U.S. as children, as well as their parents. Together, these actions could cover up to nearly 5 million people.

Further, while the Senate in June 2013 passed an immigration overhaul that includes a path to citizenship for some illegal immigrants, the House has not passed such a bill.