With the Gang of Eight’s immigration measure coming under the legislative magnifying glass this week, senators on a key committee are sharpening their red pencils in preparation to edit the 844-page bill.

The 18 members of the Senate Judiciary Committee have proposed 300 amendments to the legislation, ranging from protections for gay couples, to border-security fixes, to efforts to dismantle the bill’s central goal of creating a path to citizenship for undocumented immigrants.

Politico Playbook: "Tea party heavyweights Marco Rubio and Jim DeMint are on opposite sides of the immigration debate – and they're duking it out for the support of the movement," write Politico's Anna Palmer and Tarini Parti. John Harris joins Morning Joe to discuss.

Friends and foes of the reform effort will push their proposals starting Thursday, when the committee begins marking up the legislation. While observers do not expect that the bill will undergo dramatic changes in the committee process -- with bipartisan proponents of reform on the panel likely to stick together to resist substantial changes to their core legislation -- the high-profile debate is sure to elevate the often-dull “markup” process to must-see TV for anyone with a dog in the immigration fight.

While Republicans proposed the lion’s share of the changes -- 194 in total from the GOP side -- some Democratic amendments will be controversial as well.

As expected, Democratic Sen. Patrick Leahy, the panel’s chairman, proposed changes that would make the foreign-born same-sex partners of U.S. citizens eligible to apply for green cards. Many Republicans -- including key Gang of Eight author Sen. Marco Rubio -- have resisted the change, which some suggest would torpedo the entire bill by angering religious organizations and other social conservatives who have otherwise expressed support for the reform legislation.

That has prompted a prominent gay rights group to bluntly label opposition to Leahy’s proposal as “homophobia.”

“Labeling the inclusion of bi-national couples in the immigration bill as toxic is nothing more than a tired, insulting ruse designed to distract attention from their own failure to represent all Americans,” the Human Rights Campaign wrote in a statement.

Mandel Ngan / AFP - Getty Images file photo Sens. Patrick Leahy (R) and Chuck Grassley both have proposed amendments to the bipartisan immigration overhaul scheduled for markup in the Senate Judiciary Committee.

Many of the potential legislative additions from Republicans are designed to ensure beefed-up border security, which Rubio and others have pledged to support to help woo skeptical conservatives' support for the bill.

The border-security proposals range from changes to the timeline for plan implementation to massive influxes of additional boots on the ground. An amendment by Sen. Chuck Grassley of Iowa -- who submitted the most proposed changes with 77 total amendments -- would require the Department of Homeland Security to demonstrate “effective control” of the border for six months before undocumented immigrants become eligible to begin the process for legal status. Sen. John Cornyn of Texas submitted his own 70-page plan to step up border security and stiffen the “triggers” for the path to citizenship. Tea Party newcomer Ted Cruz -- also from Texas -- would triple the amount of federal agents currently stationed on the U.S.-Mexico border and quadruple the presence of drones and cameras.

Other proposed changes would intensify requirements for undocumented immigrants who hope to attain the bill’s new Registered Provisional Immigrant status. As written, the legislation would require individuals to pass a background check and pay back taxes and a series of fines before being eligible for legal status; some GOP amendments would increase the background check threshold, adjust the fines for inflation and aggressively enforce the bill’s existing language preventing recipients of legal status from receiving federal benefits for 10 years.

Republican Sen. Orrin Hatch of Utah even proposed that undocumented immigrants should have to submit DNA as a part of their application for provisional status in order to weed out potential criminals.

Sparks are sure to fly over the amendments that would essentially gut the pillars of the bill’s delicate compromise between immigrant groups, business organizations and labor unions.

Cruz has proposed a measure that would flatly deny eligibility for citizenship to anyone who has been “willfully present” in the United States without legal status. Those rules would apply even to undocumented immigrants who have returned to their home country or to children brought to the United States illegally by their parents – those who would be eligible for the DREAM Act provisions.

Sen. Jeff Sessions, a leading opponent of the legislation, would cap the total number of new legal permanent residents and temporary foreign workers at 30 million over a period of 10 years. He would also require the Department of Homeland Security to take into account the “likelihood” that an undocumented immigrant applying for legal status may require federal means-tested public benefits like welfare “at any point in the future.”

In a statement, Sessions said his proposed caps would preserve job opportunities for American citizens.

“This bill would authorize a dramatic surge in permanent low-skill and chain migration -- and would bring in millions more temporary foreign workers -- at a time when 90 million Americans are outside the labor force and nearly 50 million are on food stamps,” he said. “The result would be lower wages and more unemployment.”

Still more fixes are intended to focus on rooting out foreign terrorists, spurred in part by heightened concerns in the wake of the Boston Marathon bombings.

Sen. Lindsey Graham of South Carolina, a Republican member of the Gang of Eight, proposed three amendments designed to address concerns about visa security and terrorism prevention. Hatch will push for the Department of Homeland Security to collect biometric data from foreigners who are leaving the country at the nation's busiest airports.

The markup begins Thursday and is likely to continue into next week.

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