Exhibit 3.1

AMENDMENT NO. 2 TO

FIRST AMENDED AND RESTATED AGREEMENT OF LIMITED PARTNERSHIP

OF

TARGA RESOURCES PARTNERS LP

This Amendment No. 2 (this “Amendment No. 2”) to the First Amended and Restated Agreement of Limited Partnership (as amended, the “Partnership Agreement”) of Targa Resources Partners LP (the “Partnership”) is hereby adopted by Targa Resources GP LLC, a Delaware limited liability company (the “General Partner”), as general partner of the Partnership. Capitalized terms used but not defined herein are used as defined in the Partnership Agreement.

WHEREAS, the General Partner desires to amend the Partnership Agreement to make adjustments to a certain definition; and

WHEREAS, acting pursuant to the power and authority granted to it under Section 13.1(d) of the Partnership Agreement, the General Partner has determined that the following amendment to the Partnership Agreement does not require the approval of any Limited Partner.

NOW THEREFORE, the General Partner does hereby amend the Partnership Agreement as follows:

Section 1. Amendment.

(a) Section 1.1 is hereby amended to add or amend and restate the following definition:

““Available Cash” means, with respect to any Quarter ending prior to the Liquidation Date:

(a) the sum of (i) all cash and cash equivalents of the Partnership Group on hand at the end of such Quarter, and (ii) all additional cash and cash equivalents of the Partnership Group on hand immediately prior to the date of the distribution of Available Cash resulting from borrowings for working capital purposes subsequent to the end of such Quarter, less

(b) the amount of any cash reserves established by the General Partner to (i) provide for the proper conduct of the business of the Partnership Group (including reserves for future capital expenditures and for anticipated future credit needs of the Partnership Group) subsequent to such Quarter, (ii) comply with applicable law or any loan agreement, security agreement, mortgage, debt instrument or other agreement or obligation to which any Group Member is a party or by which it is bound or its assets are subject or (iii) provide funds for distributions under Section 6.4 or 6.5 in respect of any one or more of the next four Quarters; provided, however, that the General Partner may not establish cash reserves pursuant to (iii) above if the effect of such reserves would be that the Partnership is unable to distribute the Minimum Quarterly Distribution on all Common Units, plus any Cumulative Common Unit Arrearage on all Common Units, with respect to such Quarter; and, provided further, that disbursements made by a Group Member or cash reserves established, increased or reduced after the end of such Quarter but on or before the date of determination of Available Cash with respect to such Quarter shall be deemed to have been made, established, increased or reduced, for purposes of determining Available Cash, within such Quarter if the General Partner so determines.

Notwithstanding the foregoing, “Available Cash” with respect to the Quarter in which the Liquidation Date occurs and any subsequent Quarter shall equal zero.

To the extent that cash on hand is sufficient at the date of distribution of Available Cash, it will not be necessary for the Partnership to borrow for purposes of the distribution.”

Section 2. General Authority. The appropriate officers of the General Partner are hereby authorized to make such further clarifying and conforming changes to the Partnership Agreement as they deem necessary or appropriate, and to interpret the Partnership Agreement, to give effect to the intent and purpose of this Amendment No. 2.

Section 3. Ratification of Partnership Agreement. Except as expressly modified and amended herein, all of the terms and conditions of the Partnership Agreement shall remain in full force and effect.

Section 4. Governing Law. This Amendment No. 2 will be governed by and construed in accordance with the laws of the State of Delaware.

IN WITNESS WHEREOF, the General Partner has executed this Amendment No. 2 as of May 25, 2012.