Sens. Robert Menendez (D-N.J.), right, and Charles E. Schumer (D-N.Y.) in January 2013 on Capitol Hill. (J. Scott Applewhite/AP)

Six senior Democratic senators expressed support Monday for President Obama's plans to take executive action to change the nation's immigration laws, telling him to "improve as much of the immigration system as you can."

The letter was signed by Majority Leader Harry M. Reid (D-Nev.) and his three deputies — Sens. Richard J. Durbin (D-Ill.), Charles E. Schumer (D-N.Y.) and Patty Murray (D-Wash.). They were joined by Sens. Robert Menendez (D-N.J.) and Michael Bennet (D-Colo.). Durbin, Schumer, Menendez and Bennet were the four Democrats in the bipartisan "Gang of Eight" that wrote the immigration bill passed by senators last year.

"Immigrant communities have waited too long for House Republicans to catch up with the American public’s support for comprehensive immigration reform," the senators wrote. "We strongly support your plan to improve as much of the immigration system as you can within your legal authority, and will stand behind you to support changes to keep families together while continuing to enforce our immigration laws in a way that protects our national security and public safety."

The senators' message is similar to a letter sent last week by supportive House Democrats, who also urged Obama to act and said they would provide him with whatever political support needed to refute criticisms from congressional Republicans.

Obama is expected to use his executive authority in the coming days or weeks to expand temporary protections to millions of undocumented immigrants, as well as to expand visa programs for highly-skilled technology workers and perhaps also stiffen security along the U.S.-Mexico border.